The problems with transporting the large and increasing number of people living in cities are well known. Public transportation with large vehicles in the form of metro, trolley and buses all have the problem with people having to wait for vehicles to arrive and then stop at all stations during the trips. Cars offer the flexibility of a personal trip but have problems with pollution, accidents, congestion and land use. A transit system which offers the flexibility of the car without its drawbacks is widely known as PRT.
Many PRT systems have been described and patented. These systems can be characterized as having rotating motors driving on wheels or linear motors. The wheel traction based systems have problems with loss of traction in some weather conditions while the linear electric motor systems have an economic and efficiency problem, as linear electric motors are in general more expensive and less efficient than rotating electric motors. For large vehicle systems like trains the uncertain traction can be compensated with long headways, i.e. inter-vehicle times. This is not possible for a system with small vehicles as the reduced track capacity would make the system economically infeasible.
Vehicles of PRT systems can further be characterized as either supported on the track or suspended under the track. One main advantage with a suspended system is to avoid accumulation of snow, water or debris on the running surfaces of the track. A suspended system can achieve this by having only one track opening, facing downwards, greatly reducing the risk of foreign particles entering the track.
Many previous PRT systems have been designed with cabins suspended under the track. One main problem with this type of configuration is that the running surfaces on each side of the track opening must be kept at a constant lateral distance. This is structurally complicated as the track usually has a rather high U-shape internally to allow vehicles to pass. This problem is compounded by the fact that a vehicle is subjected to lateral forces acting on the cabin. These lateral forces translate to torsional moments which tend to pry the track open, i.e. to increase the width of the track opening. To avoid this the track must be made stiff, which increases its weight, cross-section size and cost.
To implement a PRT system a possibility to individually switch each vehicle to a selected track at switch points is required. Many systems having an on board switch mechanism have been designed and patented. Such designs have the advantage of allowing switching without moving parts in the track. A problem with this type of switch mechanism, particularly for a suspended vehicle configuration, is to maintain the possibility to transfer above mentioned torsional moments from the vehicle cabin to the track at all times when negotiating switches.
U.S. Pat. No. 3,830,163 describes a PRT system. A vehicle of said prior art PRT system does not have separate guide and switch wheels, which means that switching movement must be performed when the guide/switch wheels are under pressure from torsional moments, causing wear of wheels and tracks, noise and excessive energy use. In addition the system has drive wheels bearing down on an upwards facing surface of the track, an arrangement which does not provide a safe traction as stated above. Furthermore the switch mechanism of said disclosure has a downwards facing central rail which prevents the use of upwards facing drive wheels.